Worried about your civil liberties and privacy? Then it may come as a shock to discover that you have unwittingly been allowing your phone to signal your every move.

Bluetooth, a wireless link built into many cellphones, makes our movements trackable by anyone equipped with a PC and an appropriate receiver. Vassilis Kostakos at the University of Bath in the UK placed four Bluetooth receivers in the city's centre. Over four months, his team tracked 10,000 Bluetooth phones and was able to "capture and analyse people's encounters" in pubs, streets and shops.

Bluetooth is now more of a privacy threat than the more frequently publicised RFID chips, Kostakos says. "If people are worried, they should turn off the Bluetooth function on their mobile phones."